Valves and valve plugs are well known in the art. Typically, valve plug heads are positioned within the valve to control the volume of flow passing through the valve. By modifying the position of the plug head relative to the valve seat, control of the flow volume is achieved, thus allowing diversion and restriction of fluid flow. Plug heads are subjected to fluid forces, chemical attack, thermal stresses, impact from particulates and debris, as well as the forces used to attach it to the plug stem and seat loading forces that can occur when the plug head comes into contact with the valve seat. The valve head is typically attached to a plug stem, which in turn is connected to an actuating device. This actuating device is controlled to move the plug stem, which acts to change the position of the plug head to control the flow passing through the valve.
The plug stem is subjected to axial forces as the actuator moves it, mounting forces relating to the actuator attachment, and the long cylindrical section is subjected to bending forces. The plug head and the plug stem perform distinctly different purposes and are subjected to very different forces. The plug head, sitting in the middle of the flow stream, diverts and/or restricts flow, and is subjected to fluid and seat loading forces and to forces related to attaching the plug head to the plug stem. In contrast, the plug stem is moved by an actuating device to provide a sealing surface and is subject to axial and bending forces. In industrial, high volume/flow rate valves, the forces placed on plug stems and plug heads are typically significant contributors to valve failure.
Traditionally, valve plug heads are either composed of one monolithic material or make use of more than one material. Plug heads employing more than one type of material have particular advantages, in particular, better erosion and corrosion resistance, improved shock absorption, working life, and thermal expansion qualities. However, the use of a plurality of material types has been limited by the ability to effectively join the materials together economically and without creating stress points that limit the life of the plug head.
The most common methods of fixing dissimilar materials together in a valve plug are taper fitting or interference fitting, both of which employ a retaining ring that is fixed around the plug head. Taper fittings have been shown typically to subject the plug head to undesirable stresses, contribute to thermal expansion problems, and are difficult to repair. The typical taper fitting design requires a mating of two conical surfaces, one on the plug head, and the other on the retaining ring. Since neither the plug head nor the retaining ring can be manufactured with ideal cone shapes, the plug and seat may not mate perfectly. As such, loading between the two mated structures may not be uniform. Additionally, the force of the retaining ring on the plug head is exerted close to the edge of the plug head and is generally perpendicular to the angle of the conical surface. The location and angle of the force can introduce undesirable tensile forces into the portion of the plug head that bears the force. Often the desired plug head material may demonstrate weak tensile strength, thus introducing additional tensile forces that can either limit the selection of plug head materials or that can cause breaking of the edge of the plug head, separating the plug head from the plug stem and causing valve failure. Also, as the retaining ring wears away through normal corrosion and erosion, the shape of the contact area can change, typically moving closer to the edge of the plug head. This contact area change tends to concentrate forces on the edge of the plug head and increases the likelihood that the edge of the plug head will fracture, thereby causing the plug head to separate from the plug stem. The stresses induced with the taper fit are difficult to quantify and, therefore, can detract from a valve plug's performance. Variables in the welding process, such as weld shrinkage, inter-pass temperature, amperage of weld, inert gas environment, and the amount of initial burn-in, can change the amount of stress in the plug head.
As noted above, typical prior taper fit designs attach the taper fit ring to the plug stem via welding. This approach results in the retaining ring and the plug stem becoming permanently joined into one component. If the plug head wears away or breaks and the plug stem is still usable, the typical taper fit design does not lend itself to achieving the proper concentricity between the plug head and the plug stem after the plug head has been replaced. When a taper fit valve plug is repaired, the plug stem has already been machined, so it is not possible to make adjustments in the plug stem to ensure concentricity with the plug head. If the plug head is misaligned, adjustments cannot be made without cutting the taper fit ring off. For at least these reasons, taper fit valve plugs are usually discarded (as opposed to being repaired) when the plug head has broken or worn away. During assembly, the taper fit ring is typically fit tightly around the ceramic plug and the taper fit ring is welded to the plug stem. At elevated operating temperatures, the taper fit ring increases in size more than the plug head, and the plug head becomes somewhat loose in the taper fit ring, which thereby leads to early failure of the fit in operating conditions.
Interference fittings typically require a bulkier retaining ring, contributing to the load on the plug head. Interference fittings also require more complex procedures to replace plug heads and are generally limited in their service temperature ranges. An interference fit achieves more uniform loading of the plug head than does the taper fit. However, the typical interference fit uses a one-piece retaining ring that not only holds the plug head but also attaches the plug head/retaining ring assembly to the plug stem. The interference fit also must have sufficient material to allow for the wear due to erosion and corrosion without causing the plug head to separate from the plug stem. These requirements result in a bulkier retaining ring than is required to hold the plug head in place, which contributes to an additional load on the plug head. This additional load introduces tensile stresses, which tend to contribute to plug head breaking and separation, which can result in valve failure.
Even with interference fit designs, the task of replacing the plug head is quite complex. To replace the plug head, the interference fit ring must be cut, separating the plug head and ring assembly from the plug stem. This process is usually performed on a lathe or mill. If the ring is to be used again, it is necessary to separate the ring from the plug head. Certain combinations of plug head and interference fit ring materials can be separated by heating the assembly in an industrial oven. If the coefficient of thermal expansion of the retaining ring is sufficiently higher than the plug head, the retaining ring will expand more quickly and the interference fit will be negated as a space forms between the two surfaces. This approach is somewhat destructive and requires that the interference-fit ring be carefully checked before reuse. Also, this heating method only works with certain combinations of materials. Moreover, even when it may work, the plug head replacement process requires specialized manufacturing facilities that are generally unavailable to users in remote locations. Therefore, replacing plug heads for valve plugs is not a typical industry practice for certain combinations of materials or user locations.
Another problem with interference fittings is that service temperature ranges are limited because of differential thermal expansion between the plug head and ring materials. The amount of interference between the plug head and the ring is directly related to the amount of stress in a plug head. The amount of interference at ambient temperature becomes a concern when it places large amounts of stress on the plug head. Thus, when the valve plug is installed and is warming to operating temperature, the plug head is more highly stressed and is more vulnerable to failure. It has also been observed that because of these stresses, certain valve plugs, head and rings, could not be used because the ambient temperatures, or below ambient storage temperatures, could cause the plug head to fail before they could placed into service.
Also, both taper fittings and interference fittings suffer from the impracticalities of stress relieving heat affected weld zones with heat treatments. For highly corrosive fluid applications and with certain materials, it is important to stress relieve heat affected weld zones with heat treatments. With both prior existing taper fit and interference fit designs, this has not been considered practical because stress-relieving typically is performed at temperatures high enough to allow the plug head to be excessively loose in the ring, and it is not possible to assure that the plug head would return to its proper position upon cooling. Therefore, even though heat treatments might be beneficial, they have generally been avoided.
In view of the foregoing shortcomings, it would be desirable to provide a valve plug design that uses a clamping system to attach the valve plug head to its valve plug stem, and to thereby provide a means of assembling and replacing worn plug heads in the field, while allowing use of different materials for the plug head and the plug stem, where the different materials are selected specifically to address the different function of the plug head and the plug stem. This type of plug design is particularly desirable for use in flow streams that are erosive or corrosive in nature, because plug heads in these kinds of streams typically suffer material loss due to the erosion and/or corrosion and require regular replacement. Often the plug head wears out before other valve components. Therefore, minimizing the occurrences when the plug head fails and must be replaced is very desirable in improving the life cycle and efficiency of the valve.